The present invention relates to a configuration and an operation method of a passive optical network system in which a plurality of subscriber connecting units shares an optical transmission line.
Since the communication speed is increasing and the communication range is expanding also in access networks to connect subscribers to communication networks, it has been considered to introduce a Passive Optical Network system (to be referred to as PON system hereinbelow) prescribed by, for example, recommendations of International Telecommunications Union-Telecommunication Standardization Sector (to be referred to as ITU-T hereinbelow; ITU-T Recommendation G.984.1, G.984.2, G.984.3). The PON system is a system in which an Optical Line Termination (to be referred to as OLT hereinbelow) to be connected to a higher-level communication network is connected to an Optical Network Unit (to be referred to as ONU hereinbelow) to accommodate a plurality of subscribers' terminals (Personal Computers (PC) and telephones) by use of a passive optical network including a basic trunk optical fiber line, an optical splitter, and a plurality of branch optical fiber lines. Specifically, signals from terminals (e.g., PC) connected to the respective ONU are sent in the form of optical signals via branch optical fiber lines. The optical signals are optically multiplexed using time division by the optical splitter on the basic trunk optical fiber to be sent to the OLT. The OLT receive the signals from the respective ONU to process the signals and then transmit the signals to an access/core network behind the OLT (to be referred to as access/core network hereinbelow) or to a second ONU connected to the pertinent ONU.
In the introduction of PON systems, a relatively low-speed system which operates at 64 kilobits per second (Kbps) is introduced in the initial stage. At present, a relatively high-speed system operating at a high transmission speed of about 2.4 Gigabits per second (Gbps) standardized according to ITU-T recommendations is being increasingly introduced. In addition, it is desired to implement in the future a higher-speed PON system capable of handling signals of a transmission bit rate ranging from 10 Gbps to 40 Gbps. To realize such high-speed PON system, there have been discussed various multiplexing methods such as the Time Division Multiplexing (TDM) method to divide a large number signals with respect to time and a Wavelength Division Multiplexing (WDM) method to divide a lot of signals with respect to the wavelength. Primarily, adoption of the TDM method is being discussed in consideration of affinity or and compatibility thereof to existing PON systems.
Additionally, discussion has been started for a PON system including a lower-speed PON and a higher-speed PON in which the transmission speed of the system is sequentially increased. That is, discussion has been conducted also for a configuration and an operation method of such PON system in which a plurality of PON systems coexist by use of a time-division multiplexing method using one and the same wavelength in terms of optical characteristics. To provide inexpensive and less power consuming access networks, there have been proposed various PON system configurations and operation methods such as a method of increasing the number of subscribers to be accommodated, a method of elongating the transmission distance, and a method of increasing the transmission speed. Also, improvement of characteristics of associated devices such as an optical amplifier and transceiver has been discussed. For example, to implement data transmission at 10 Gbps by use of existing optical fiber lines (32 or 64 branch lines; transmission density of 20 km), the Full Service Access Network (FSAN) as a standardization organization is discussing adoption of a semiconductor optical amplifier (SOA or Erbium-Doped Fiber Amplifier (EDFA), Praseodymium-Doped Fiber Amplifier (PDFA)); an electro-absorption modulator integrated distributed feedback laser (EA/DFB), a highly-sensitive receiver (Avalanche Photodiode (APD)), Forward Error Correction (FEC), and a dispersion compensating function for optical electric signals.